I did nothing to my computer, all I did was shutdown and then turn it on the next day and when I boot up it gave me this error:

DISK BOOT FAILURE INSERT SYSTEM DISK AND PRESS ENTER

What do I do?

Put in your OS CD and reinstall the OS, not reformat, just reinstall...

Do you have a floppy disk in the drive?

Yeah, that would be the problem as well. lol

Or an overheated/bad powered harddrive that cools down and doesn´t see the bootsector any more, I saw it more times here...

WELL, first you have to understand what your computer is telling you with that message.

1. It's telling you that on the drive that you have told your Bios to use as your boot drive, its not finding the boot sector. Or, the Bios can't find the drive at all.
2. See iten #1.

So why is it not seeing a boot sector? Having a floppy disk in the FD that does not contain boot information could be the cause of the problem. Make sure there is NO disk in any drive, floppy or CD.

Then Look at your bios by pressing Del or the appropritate key while you system is booting up, from a cold start.

If the time and date are wrong in the first Bios screen, that's a dead givaway that your CMOS battery had failed. They are only spec'd for about four years of use.
They actually can fail at any time. Replacing the CMOS battery is usually pretty easy and the batteries (CR-2032) can be had at any drug store, or discount store, or Radio Shack.

So the bottom line is,,,,,
If your removable drives are empty of disks and your Bios is working and set to boot from your HD, then you can be pretty well assured that the problem lies in the drive itself.

tubut,
you gave us NO information about your sytem so going any further in trying to troubleshoot your problem is fruitless.

Another possible source of the problem (and from reading your description of how it occured is probably most logical) is that you deleted your boot.ini and ntldr. I had a friend who's comp I worked on. She found a bunch of files "I had left laying around on the C drive", and decided it would be a good idea to delete them. Basicaly, I turned on Show Hidden files, and turned off Hide Operating system files, and she delted them. Try a re-install, and you should be fine.

If you delete NTLDR it'll actually say "NTLDR is missing." (Try it sometime, just remember the fixboot command from recovery console...)

The "DISK BOOT FAILURE" looks more like a hardware error.

1. The boot sector, possibly the partition was wiped out or corrupted by malware.

2. Bad HDD.